Depletion of crude oil and natural gas has been a concern in recent years. Oil fields and gas fields located deeper than conventional fields and known as “deep-water” oil fields and gas fields located 2000 m under the sea or deeper are now being exploited. Deep-water oil fields and gas fields operate in severe corrosion environments that contain CO2, H2S, Cl−, and the like in high concentrations. Accordingly, steel materials used in such oil fields and gas fields are required to exhibit high strength and high corrosion resistance.
Crude oil or natural gas mined from an oil field or a gas field through oil country tubular goods (OCTG) or the like is transported to ground (or sea base) facilities through gathering lines laid along the ocean floor. Accordingly, the line pipes and other components used are required to exhibit strength that can withstand water pressure and an ability to sink by their own weights. In this regard, the wall thickness of the line pipes used exceeds 20 mm and in some cases heavy wall seamless steel pipes having a thickness of 35 mm are used. These heavy wall seamless pipes are required to exhibit higher strength, high corrosion resistance, and a good girth welding property.
To meet these requirements, for example, Patent Literature 1 and Patent Literature 2 describe a high-strength seamless steel pipe that has a composition including C: 0.03 to 0.11%, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.025% or less, S: 0.003% or less, Ti: 0.002 to 0.017%, Al: 0.001 to 0.10%, Cr: 0.05 to 0.5%, Mo: 0.02 to 0.3%, V: 0.02 to 0.20%, Ca: 0.0005 to 0.005%, N: 0.008% or less, and O: 0.004% or less and has a bainite and/or martensite microstructure with ferrite precipitating in grain boundaries. According to a technique described in Patent Literature 1, a steel slab having the aforementioned composition is formed into a seamless steel pipe by hot rolling. The seamless steel pipe is subjected to a quenching treatment of cooling the seamless steel pipe at a cooling rate of 5° C./s or more from a quenching start temperature of (Ar3 point+50° C.) to 1100° C., and then to tempering at 550° C. to Ac1 point so as to form a high-strength seamless steel pipe that has a yield strength of 483 MPa or more and excellent hydrogen induced cracking resistance (hereinafter also referred to as HIC resistance).
Patent Literature 3 describes a method for producing a heavy wall seamless steel pipe for line pipes that have high strength and good toughness. The technique described in Patent Literature 3 sequentially performs the following steps: a step of causing a molten steel to solidify into a billet having a round cross-section by continuous casting, the molten steel containing C: 0.03 to 0.08%, Si: 0.25% or less, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, at least one of Ca, Mg, and REM: 0.0002 to 0.005% in total, V: 0 to 0.08%, Nb: 0 to 0.05%, and Cu: 0 to 1.0%; a step of cooling the billet to room temperature at an average cooling rate from 1400° C. to 1000° C. of 6° C./min or more; a step of heating the billet to 1150° C. to 1280° C. at an average heating rate from 550° C. to 900° C. of 15° C./min or less and preparing a seamless steel pipe by piercing and rolling; a step of continuously conducting forced cooling to a temperature of 100° C. or lower at an average cooling rate from 800 to 500° C. of 8° C./sec or more after the seamless steel pipe was soaked at 850 to 1000° C. immediately after preparation of the seamless steel pipe, after the seamless steel pipe was cooled after the preparation and then heated to 850 to 1000° C., or immediately after preparation of the seamless steel pipe; and a tempering process of tempering the pipe at a temperature in the range of 500° C. to 690° C. According to the technique described in Patent Literature 3, a rectangular bloom or slab may be formed by continuous casting instead of the round billet and then formed into a round billet by forging or rolling.